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I'll try once more. What size batteries do yo have? How many 120 VAC running amps does the A/C you intend to run draw,and how many amps do any other 120 VAC loads use at the same time as the A/C? Then add up all the 12 VDC amps that will be used (lights, radio, etc). How large is the inverter you intend to use and its efficiency rating? How many amps does your generator put out? You can determine the generator output by taking the bus to a battery store with a large enough load tester and have them keep adding load until the DC ammeter stops rising. If you give us all those numbers someone can tell you if your idea will work.

The original 4104 blower motor was rated at 3/4 horsepower and only pulled around 50 amps at high speed. The total DC load at full air conditioning was about 60 amps including the defrost blowers. A 2800 watt inverter is going to pull over 200 amps at load.

The point is, that while the 4104 was originally designed to operate the air conditioner from the bus system, it is no where near the capacity we consider normal today.

I will echo the others here; use a generator. It is far less expensive than trying to run air conditioning from the bus. In hot Florida like climate, you will need more than one roof air to keep up with the sun on the windshield. In addition, the 4104 is under powered to begin with, you will be better off not using the engine for those loads.

goodnews, I think a good quality inverter running a single AC is within reach, and you have a good idea what it will take to overcome the positive ground issue.

As for the math, the battery bank will take care of start currents, that's their job.

The running current will be somewhere 11-12 amps on a typical roof air, which you multiply by 10 to get the amps needed at 12 volts for the inverter. So, for this conversation, we go big and say 120 amps.

Then you have to add the efficiency loss of the inverter you choose, which even if it was a crappy 80% unit, would be another 24 amps or so in inverter waste. This number goes down if you select a better inverter. There are lots to choose from on the internet. It requires careful inspection of the technical specifics, and at what amperage the efficiency was measured. Inverters are not consistently efficient across their operating range. Once the selection process has been narrowed down, a call to the manufacturer to clarify would be time well spent.

So, your question for the AC issue is whether the charging system in your 4104 can reliably produce a continuous 145 amps, just for the AC install, or a bit less, depending on your exact numbers?

And, before we're done, you need more than that to run the rest of the loads and to charge the batteries.

If the batteries are only used to start the bus and support the chassis while underway, they won't need much charging. If those batteries get used while camping, without a charger on them, they will need more charging while underway.

Running off an inverter, a rough rule of thumb is to take the running amps of the 120vac and multiply it by 11 to include efficiency losses. Hence a 15 amp 120vac running load, like an A/C would be drawing 165 amps/hour from the batteries. So to run an A/C all night on batteries for 8 hours (if the compressor is running continuously-like in over 100 degree weather in summer [it will stay over 100 at night in the desert]), you would draw 1320 amp/hours. An 8D Lifeline AGM battery has 255 amp/hours and then you don't want to run it down past 50%-for a usable 130 amp/hours. Round figures, you'd need 10-8D batteries-or 1650lbs worth of batteries! In this case-just run the generator.

I have 2 Lifeline AGM 8D batteries (that I just bought-the old ones went almost 7 years) for 255 amp/hours usable at 50% discharge. My refrigerator draws 5 amps, but runs about 50% of the time-so that's 2.5 amp/hours. Then at night, I generally pull around 20 amps of lighting for 6 hours-another 120amps. Basically, when dry camping, my 2-8D's will get me through the night comfortably running lights, TV. Then in the morning, fire up the generator to warm up the two water heaters, brew coffee, run the bathroom heater, charge the batteries for two hours. Good Luck, TomC

I use an inverter to run my roof-top on the road, and I'd be perfectly comfortable with just my two 8D start batteries if need be, but as noted you have to be careful of the charging steady state output capacity of your alternator/charging system, and you have to be real careful not to run the AC from the batteries if the engine is off. In my case, I have a 24 volt system with the 50DN, so I have a different situation than you do.

Inverters are rated Example :4000=surge but are only continous 3200 watts Less 20% efficiency or about 25 amps start up and 18 running at max battery/alt performance. would prob pull batteries below starting capacity with engine off in less than 1/2 hr. Don't know about idle or slow traffic. If your is same post that was on other board and you have a 15000 roof top your power load should be up toward the 18 amp start up and 14 amp running. The older your units get; normally power demands go up. Not trying to talk you out of it!!!! Just be aware. A Harborfreight unit is not going to do it. Read the actuall output continous of whatever you buy. Do math gen set fuel vs cost of all the inverters etc and possible damage to AC. If your not the same guy that posted over at BNO sorry about confusion. Do experiment!::: but at least little sign of trouble have a back up plan. If you take nothing else from these comments have a backup as not to ruin your trip and bussin experience. Bob

You're already running a fairly low power engine to start with. Power from an engine driven alternator/generator is not free. The more amps you demand from the alternator the more power it uses from the engine. You probably need all the power the engine can provide when going up a grade.

The cost of an inverter can probably pay for about 500 hours worth of fuel for a generator if you already have a generator.

You can certainly do what you want, you have to decide if the cost and loss of engine power is worth it.

On our 4104, we ran the generator when we needed AC. We installed a small inverter to run small items. Because of limited space for more batteries and being a 12 volt system, this was the simplest setup. If the two 8d's got too low to start coach (rarely), we would start generator to recharge batteries. It would take a long time to recover savings from adding a large inverter, more batteries, and bigger alternator. Running ac's off engine instead of generator. Engine mileage would drop some, maybe close to generator consumption. I would go to the house battery/inverter setup on a 40footer with 24 volt system, maybe depending on how much it is actually used.You could use the generator to charge the house batteries while it is running to take load off of coach engine....many possibilities. Anymore, many owners aren't going to put the miles on a conversion like they did 5 or more years ago, when diesel was $2 a gallon.

I just seem to have lost a post. Will try again. I really appreciate all the help. I have had to miss the conversation for a couple of days but wanted to let you know that all the information has been wonderful and I am still trying to look at all options.

Which brings me to ask if any of you 3104 owners have mounted a regular automotive air compressor on your 671 inline engine? I have a a unit the I am thinking of mounting on the engine and running freon lines to the cooling coil in the cabin for down the road air but do not see a good place to mount it on the engine.

If any of yiou have done this and could share some information and perhaps a photo of the install, I would greatly appreciate it. The unit I am talking about is belt driven.